1. Field of the Invention
The present invention relates to an image processing apparatus and an image processing method for performing digital processing on an input image and outputting density data to an output apparatus.
2. Description of the Related Art
Generally, digital image processing apparatuses such as digital copying apparatuses that reproduce images by outputting digitized image data from digital printers, such as laser beam printers, have become widespread instead of conventional analog image processing apparatuses due to the development of digital equipment. These digital image processing apparatuses generally adopt a method for performing tone reproduction through halftone processing, such as processing using a dither processing method, in order to reproduce halftones.
Although tone reproduction using a dither processing method is favorable for a portion having few high frequency components as with a flat portion, there has been a problem in that breaks called jaggies occur in a character/thin line portion. This is due to a cycle that a dither has interfering with high frequency components included in an input image (in particular, a cyclic pattern similar to the dither cycle) so as to cause a cyclic fringe pattern called a moire phenomenon to occur.
To address this, there is an error diffusion method as a tone reproduction technique other than the dither processing method. This technique is a technique in which the density difference (quantization error) between a pixel density of input image data and an output pixel density is calculated for each pixel, and after performing specific weighting on the quantization error, which is the result of this calculation, the quantization error is diffused to the peripheral pixels of a pixel of interest (e.g., see “An Adaptive Algorithm for Spatial Grey Scale” in Society for Information Display 1975 Symposium Digest of Technical Papers, 1975, p. 36 (Document 1)). This technique makes it possible to obtain an output image that does not have cyclicity, and has a high resolution, and accordingly the moire phenomenon and jaggies in a character/thin line portion do not occur.
However, this error diffusion method has problems, such as a distinctive fringe pattern (texture) occurring in an output image, or granular noise being noticeable in the highlight portion and dark portion of an image in electrophotographs. As a technique for suppressing image deterioration that occurs with this dither processing method and error diffusion method, and precisely reproducing a high quality image, there is a method in which tone reproduction is performed on input images using a dither processing method and an error diffusion method, the processing is switched based on the type of the images, and the resultant images are outputted (e.g., see Japanese Patent No. 2621865 (Document 2)).
However, as with the technology disclosed in Document 2, in the processing for switching between dither processing and error diffusion processing based on a threshold value, due to slight density variations in image data, processing may be frequently switched in the vicinity of a boundary (a portion in which a determination value is close to the threshold value), which causes deterioration of image quality. Further, since a dot structure changes drastically in a portion in which processing is switched, a sense of unnaturalness may be given particularly in a case of nature images. Although it is possible to cope with these problems by setting appropriate threshold values, it is difficult to set appropriate threshold values for the entire inputted image, thus requiring the addition of complicated processing.